BACKGROUND: In patients with ST-segment elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PCI) with thrombus aspiration, the histopathology of aspirated thrombus was previously related to long-term mortality. In this study, we sought to investigate the association between histopathology of aspirated thrombus and ST-segment recovery, a marker of microvascular dysfunction, immediately at the end of the PCI procedure.METHODS: We included 892 STEMI patients who underwent primary PCI with routine thrombus aspiration and for whom combined data on histopathology of aspirated thrombus and ST-segment recovery were available. Patients were categorized according to histopathology of aspirated thrombus: fresh only (<1 day), older (>1 day), or no material aspirated. ST-segment recovery was defined as incomplete if <50%.RESULTS: Incomplete ST-segment recovery occurred in 134 of 363 patients (37%) with fresh thrombus, in 104 of 238 patients (44%) with older thrombus, and in 142 of 291 patients (49%) with no material. Unadjusted odds ratios for incomplete ST-segment recovery of patients with older thrombus and no material, when compared with patients with fresh thrombus, were 1.33 (95% CI, 0.95-1.85; P = 0.097) and 1.63 (95% CI 1.19-2.23; P = 0.002), respectively. Both associations were unchanged after multivariable adjustment for clinical predictors of ST-segment recovery. ST-segment recovery was a strong predictor of long-term mortality, independent of the histopathology of aspirated thrombus.CONCLUSIONS: This study shows that ST-segment recovery immediately at the end of the PCI procedure was a significant prognosticator, independent of the histopathology of aspirated thrombus. We found that the histopathology of aspirated thrombus (fresh, older, no material) was associated with ST-segment recovery in STEMI patients undergoing primary PCI with thrombus aspiration.